Cellular changes in the lymphoreticular tissues of C57L/J mice infected with Echinococcus multilocularis cysts.

The pathology of the spleen, lymph nodes and thymus of C57L/J mice, infected intraperitoneally with 20 and 100 cysts of Echinococcus multilocularis is described at 2, 4, 8 and 12 weeks postinfection (p.i.). For the first 8 weeks, growth of the larval cyst mass (LCM) was slow and blastogenesis in T-dependent areas of both spleen and lymph nodes was moderate whereas in B-cell compartments it was intense. A rapid growth of the LCM between 8 and 12 weeks p.i., 15-20 times greater than for the first 8 weeks, was associated with depletion of lymphocytes in thymus dependent area (TDA) of both spleen and lymph nodes, gross expansion of the red pulp with extramedullary haemopoiesis, partial atrophy of spleen follicles, but not those of the lymph node, and involution of the thymus. At 12 weeks p.i. the TDA had mainly plasma cells and histiocytes among occasional lymphocytes and blast cells; germinal centre activity and plasmacytosis persisted in B-cell areas. Morphological aspects of these changes are discussed in relation to the invasiveness and proliferation of the LCM during the course of infection.     

Selective labeling of mesenteric lymph nodes: cell production and emigration of newly formed lymphocytes to other organs

Lymphocyte production by mesenteric lymph nodes of normal young pigs was studied by intranodal injections of either tritiated thymidine or tritiated deoxycytidine as DNA precursors. One or two days after selective labeling of the mesenteric lymph nodes the relative and absolute number of lymphocytes derived from mesenteric lymph nodes were determined autoradiographically in the following organs: mesenteric, cervical and inguinal lymph nodes, spleen, thymus, bone marrow, Peyer's patches, tonsil, different regions of the gut, lung and liver. The overall cell production of mesenteric lymph nodes, as derived from the sum of all labeled cells one day after labeling, was estimated to be about 7 X 10(9) lymphocytes. Up to 40% of all newly formed lymphocytes had already left the lymph nodes within one day and were found in all organs studied. There was a preferential homing to the mucosa of the small intestine, but a considerable number migrated to the spleen and even to the thymus and bone marrow. In lymphoid organs all labeled cells were small and medium-sized lymphocytes one and two days after labeling. In cervical lymph nodes, spleen, tonsil and Peyer's patches the relative distribution to T and B cell areas was determined. There was an obvious preference of newly formed lymph node cells to home to T cell areas. The differences of labeling between thymidine or deoxycytidine were surprisingly low.     

Entamoeba histolytica: Antibody responses and lymphoreticular changes in gerbils (Meriones unguiculatus) in response to experimental liver abscess and amebic extract injection

Antibody responses and histological changes in hepatic lymph nodes and spleen of gerbils (Meriones unguiculatus) during the course of experimental hepatic amebiasis (5–60 days), or in those injected with extracts ofEntamoeba histolytica, are described. Lymph node and spleen responses in infected animals paralleled the proliferation of the amebic liver abscess. However, spleen follicle responses were similar in animals that received low or high doses of the amebic extract and differed histologically from those with amebic liver abscess. Liver abscesses, up to 30 days postinfection (pi), doubled in weight between 10 and 15 and between 20 and 30 days pi. Early changes (10 days pi) in the lymphoreticular tissues were characterized by increased size and weight of the organs, hyperplastic follicles, and blastogenesis in the T-dependent areas. At 20 and 30 days pi, the size of spleen follicles increased and there was depletion of lymphocytes from the periarterial area (PAA), as well as gross extension of the red pulp, accompanied by extramedullary erythropoiesis and megakaryocytosis. The paracortical areas (PCA) of lymph nodes were depleted of lymphocytes and histocytosis throughout the organ, and there was intense plasma cell activity in the medulla. At 60 days pi, lymphocyte repopulation was noted in the PCA and PAA; germinal centers were depleted of blast cells and the spleen red pulp had contracted. Antiamebic antibody titers were low throughout the infection. Changes in the cellularity of the lymphoid organs are discussed in relation to the proliferation of the amebic liver abscesses in infected animals and in those which were injected with the amebic extract.     

Selective labeling of mesenteric lymph nodes: Cell production and emigration ofnewly formed lymphocytes to other organs

Lymphocyte production by mesenteric lymph nodes of normal young pigs was studied by intranodal injections of either tritiated thymidine or tritiated deoxycytidine as DNA precursors. One or two days after selective labeling of the mesenteric lymph nodes the relative and absolute number of lymphocytes derived from mesenteric lymph nodes were determined autoradiographically in the following organs: mesenteric, cervical and inguinal lymph nodes, spleen, thymus, bone marrow, peyer'spatches, tonsil, different regions of the gut, lung and liver. The overall cell production of mesenteric lymph nodes, as derived from the sum of all labeled cells one day after labeling, was estimated to be about 7 × 10⁹ lymphocytes. Up to 40% of all newly formed lymphocytes had already left the lymph nodes within one day and were found in all organs studied. There was a preferential homing to the mucosa of the small intestine, but a considerable number migratedto the spleen and even to the thymus and bone marrow. In lymphoid organs all labeled cells were small and medium-sized lymphocytes one and two days after labeling. In cervical lymph nodes, spleen, tonsil and peyer's patches the relative distribution to T and B cell areas was determined. There was an obvious preference of newly formed lymph node cells to home to T cell areas. The differences of labeling between thymidine or deoxycytidine were surprisingly low.     

Effect of Cyclophosphamide on T-Lymphocyte Marker Expression in T-Cell Areas of Some Lymphoid Organs of the Rat

Surface markers on rat lymphocytes from thymus and T-cell areas of lymph node, spleen and Peyer's patches were examified in histological sections after one sinale dose of Cyclophosphamide (CY, 100 mg/kg body weight). A panel of monoclonal antbodies was used: W3/13, W3/25 and OX8 to investigate Pan T, T_H and T_s/_c marker expressions respectively. T_s/_c marker expression showed a marked and significant reduction in both thymus and lymph node lymphocytes 3 days after CY treatment. This was followed by return to normal in the thymus and a significant rebound increase in the lymph node by day 7 after treatment. This T_S/c marker expression in both the spleen and Peyer's patches showed a significant increase on both day 3 and 7 after CY treatment.

Mast cells were observed in lerge numbers in the thymus and lymph node but not in the spleen and Peyer's patches. T_H marker expression was increased significantly in lymph nodes, spleen and Peyer's patches. No change was observed in Pan T marker expression in any of the tissues at any of the times examined.     

Maturation of mitogenic response in foetal guinea-pig.

Lymphocytes obtained from thymus, spleen and heart blood of 33-day-old guinea-pig foetuses and from lymph nodes of 48-day-old foetuses onwards were stimulated by phytohaemagglutinin (PHA). concanavalin A (Con A) and dextran sulphate (DxS). The results, based on the analysis of ninety-five foetuses, indicated that the mitogenic responses of the guinea-pig T cells matured first in the thymus and then in the spleen and lymph nodes, in that order. The PHA and Con A responses in the thymus, spleen and lymph nodes and the DxS response in the spleen improved with the increasing age of the foetus. A clear improvement of the mitogenic responses did not take place in the blood. Thus, at birth the guinea-pig is immunologically mature as regards the PHA and Con A responses in the thymus, spleen and lymph nodes and the DxS response in the spleen. Regarding the maturation of PHA and Con A responses by lymphocytes from the blood, further studies are needed, since at birth both of these responses are clearly below the adult level.     

Effect of Cyclophosphamide on T-Lymphocyte Marker Expression in T-Cell Areas of Some Lymphoid Organs of the Rat

Abstract

Surface markers on rat lymphocytes from thymus and T-cell areas of lymph node, spleen and Peyer's patches were examified in histological sections after one sinale dose of Cyclophosphamide (CY, 100 mg/kg body weight). A panel of monoclonal antbodies was used: W3/13, W3/25 and OX8 to investigate Pan T, T_H and T_s/_c marker expressions respectively. T_s/_c marker expression showed a marked and significant reduction in both thymus and lymph node lymphocytes 3 days after CY treatment. This was followed by return to normal in the thymus and a significant rebound increase in the lymph node by day 7 after treatment. This T_S/c marker expression in both the spleen and Peyer's patches showed a significant increase on both day 3 and 7 after CY treatment.

Mast cells were observed in lerge numbers in the thymus and lymph node but not in the spleen and Peyer's patches. T_H marker expression was increased significantly in lymph nodes, spleen and Peyer's patches. No change was observed in Pan T marker expression in any of the tissues at any of the times examined.     

Proliferation and emigration of newly formed lymphocytes from pig spleens during an immune response

Normal young pigs were immunized intravenously with sheep red blood cells (SRBC). At various times after a second SRBC injection the spleens were connected to an extracorporeal perfusion system, and proliferating lymphoid cells in the spleens were selectively labelled with tritiated thymidine. One day later the relative and absolute numbers of spleen-derived lymphocytes were determined by autoradiography in the following organs: various parts of the spleen, mesenteric and cervical lymph nodes, thymus, bone marrow, Peyer's patches, tonsils, intestine, lung, liver and blood. From 1 to 7 days after the second SRBC injection, the spleens produced increasing numbers of lymphocytes, and labelled cells were found especially in the blood and bone marrow. The newly formed splenic lymphocytes migrated preferentially to T- but also to B-cell areas in lymph nodes, Peyer's patches and tonsils. In all organs outside the spleen nearly all labelled spleen-derived lymphocytes were small lymphocytes. However, the bone marrow contained a high proportion of labelled immature and mature plasma cells. The spleen produced large numbers of lymphocytes during the secondary immune response, many of which migrated to different organs probably as memory cells, while others were found in the bone marrow as effector cells from the immune response.     

Changes produced by pertussis antigen on the blood cells and lympho-haemapoietic tissues after early and late thymectomy or splenectomy

The effect of thymectomy and splenectomy in C3H/Bi mice on the responses of circulating leucocytes and on morphological changes of the haematopoietic tissues after injection of pertussis vaccine has been studied.

After pertussis all mice showed depletion of lymphoid cells in all the lymphoid organs as well as in bone-marrow and an increased number of leucocytes, lymphocytes, neutrophils and monocytes in the circulation. Neonatal thymectomy decreased lymphocytosis produced by pertussis. Thymectomy, at all ages studied, fostered an increase in the number of monocytes and polymorphonuclears in circulation. Splenectomy at birth or early in life provoked an increase in levels of circulating polymorphonuclears and lymphocytes in pertussis treated animals.

In neonatally thymectomized mice the depletion of lymphoid cells from lymphoid tissues after pertussis could be shown to include the thymic-independent areas. The depletion of small lymphocytes from thymus following pertussis persisted longer than depletion of small lymphocytes from spleen, marrow or lymph nodes. The longer persistence of lymphoid depletion in the thymus than in peripheral lymphoid tissues is, we believe, to be related to the central lymphoid function of thymus as a site of differentiation of lymphoid cells and to the aloofness of thymus from recirculation of fully differentiated peripheral lymphocytes.

     

The effect of cyclophosphamide in vivo on the expression of lymphocyte markers, detected by monoclonal antibodies, in the rat

Surface markers on lymphocytes from the thymus, lymph node and spleen of the rat were examined in single cell suspensions using a panel of monoclonal antibodies. These were used particularly to investigate Pan T, TH (T-helper), TS/C (T-suppressor-cytotoxic) and Ia antigens. The expression of these markers in rats treated with a single dose of cyclophosphamide (100 mg/kg body weight) was followed for 3 weeks after treatment. Maximum changes were detected at 3 days and recovery took up to 3 weeks to near completion. Comparison was made with histological observations of the effect of CY on these organs. At 3 days, the thymus showed maximum weight loss and gross cortical depletion. This associated with significant drop in the expression of the TS/C marker on small and large lymphocytes. Regeneration of the cortex, beginning at 7 days, was associated with the presence of many large pyroninophilic cells. This was accompanied by an increase in the expression of the TS/C marker on both small and large lymphocytes. The mesenteric lymph node showed marked depletion of the B-cell areas at 3 days. There was also a significant drop in TS/C and Ia expression and a marked rise in Pan T and TH. TS/C expression recovered rapidly with a rebound at 7 days. However, the expression of Pan T and TH did not return to normal until 21 days. In the spleen there was a similar decrease in the lymphocytes populating the B-cell areas at 3 and 7 days with an increase in the expression of Pan T and TH, and a decrease in the expression of Ia.(ABSTRACT TRUNCATED AT 250 WORDS)     

Organ distribution and fate of newly formed splenic lymphocytes in the pig

The production of lymphoid cells in the pig spleen was studied autoradiographically after selective labeling of the spleen using an extracorporeal perfusion circuit. Tritiated thymidine was added as a DNA precursor. One to 4 days after local labeling of the spleen the relative and absolute number of spleenderived lymphocytes were determined in the following organs: mesenteric, cervical and inguinal lymph nodes, thymus, bone marrow, Peyer's patches, tonsils, three different parts of the gut, lung, liver, and blood. The labeled lymphocytes which migrated to these organs were all small lymphocytes, except for some large cells in the lamina propria. In the bone marrow, however, a considerable number of the spleen-derived immigrants were transformed into plasma cells. The total number of labeled lymphocytes decreased dramatically from Day 1 to Day 4 after labeling, indicating a high percentage of short-lived cells. Within the spleen, plasma cells had the highest labeling index of about 30% at Day 1 but this dropped to only 1.5% on Day 3. The organ distribution of the splenic emigrants changed from Day 1 to Day 4 with a relative increase in lymphocytes found in lymph nodes and a decrease in the lung and the intestinal wall. The newly formed splenic lymphocytes migrated to T- and B-cell areas in lymph nodes, Peyer's patches, and tonsils. In the intestinal wall labeled lymphocytes were found in the lamina propria and also as intraepithelial lymphocytes. There was no obvious redistribution between organ compartments with time after labeling of the spleen. The spleen produces large numbers of lymphocytes, which show typical organ distribution and homing to areas in lymphoid and nonlymphoid organs.     

Repopulation of lymph nodes and spleen in thymus chimeras after lethal irradiation and bone marrow transplantation: dependence on the age of the thymus

CAP and Lewis rats were thymectomized and received a syngeneic thymus graft followed by lethal irradiation and syngeneic bone marrow transplantation. In three groups (A: recipient 15 months old, thymus graft 3 months old; B: recipient 3 months old, thymus graft 15 months old; C: recipient and thymus graft both 3 months old), we performed an immunohistologic analysis of the splenic white and red pulp and the paracortical zone of the lymph nodes. The repopulation of these regions was demonstrated with monoclonal antibodies that react with Thy-1 positive cells, peripheral T cells, T helper cells, and T non-helper cells. In the splenic red pulp, more Thy-1 positive lymphocytes were found in group B than in group C. The proportion of T lymphocytes and T helper lymphocytes in the region of the periarteriolar lymphocyte sheath of the splenic white pulp was higher when a young thymus was transplanted (groups A and C) than when an old one was (group B). In contrast, in the splenic red pulp, more T lymphocytes were found in group A than in groups B and C. In the paracortical zone of the lymph nodes, this was demonstrable only for group C versus group B. The proportion of T non-helper lymphocytes in the region of the splenic red pulp was higher in group B than in group C. These results indicate that the repopulation of lymph nodes and spleen after transplantation of an old thymus is delayed, quantitatively reduced, and qualitatively different (more T non-helper lymphocytes).     

A secondary immune deficiency in the Fatty/Orl-op rat.

The response of T and B lymphocytes to the mitogens PHA and LPS was studied in the Fatty/Orl-op rat. Whereas T and B cells from op/op spleen showed no lack of responsiveness compared with that of normal (op/+) littermate rats, the response of thymic cells to the T cell mitogen was lower in op/op rats over 12 days of age compared with normal rats of the same age. Osteopetrotic rats of 10 days and younger did not show this T cell deficiency. The responsiveness of T cells from op/op lymph nodes to PHA was less than that of normal rats at 12 days and older. Thus, though splenic T and B cell populations were well maintained in the adult op/op rat despite the severe depletion of marrow cells in this mutant, the thymus and lymph node T cell populations were qualitatively or quantitatively deficient. Since osteopetrotic obliteration of the marrow cavities precedes the appearance of the immune deficiency we suggest that the immune deficiency may be caused by a failure of T cell supply to the thymus and lymph nodes.     

A secondary immune deficiency in the Fatty/Orl-op rat

The response of T and B lymphocytes to the mitogens PHA and LPS was studied in the Fatty/Orl-op rat. Whereas T and B cells from op/op spleen showed no lack of responsiveness compared with that of normal (op/+) littermate rats, the response of thymic cells to the T cell mitogen was lower in op/op rats over 12 days of age compared with normal rats of the same age. Osteopetrotic rats of 10 days and younger did not show this T cell deficiency. The responsiveness of T cells from op/op lymph nodes to PHA was less than that of normal rats at 12 days and older. Thus, though splenic T and B cell populations were well maintained in the adult op/op rat despite the severe depletion of marrow cells in this mutant, the thymus and lymph node T cell populations were qualitatively or quantitatively deficient. Since osteopetrotic obliteration of the marrow cavities precedes the appearance of the immune deficiency we suggest that the immune deficiency may be caused by a failure of T cell supply to the thymus and lymph nodes.     

The role of the thymus in development of lympho-hemopoietic tissues. The effect of thymectomy on development of blood cells,bone-marrow,spleen, and lymph nodes

The effect of thymectomy at different ages in C3H mice on development of circulating leukocytes and cells of marrow, spleen and lymph nodes has been analyzed. Regardless of the age at which thymectomy is performed depression of numbers of circulating lymphocytes is produced. Thymectomy at birth did not affect significantly the relative number of lymphocytes in the marrow during the first few weeks of life, later they fell to low levels. Thymectomy at four weeks was followed by prompt reduction in relative numbers of lymphocytes in the marrow. After reaching six weeks of age, neonatally thymectomized mice showed a high proportion of monocytes in the marrow. Neonatal thymectomy and thymectomy at two weeks of age reduced the number of eosinophils in the marrow. Neonatal thymectomy inhibited development of lymphocytes in the spleen, whereas thymectomy later in life produced only transient depression of lymphocytes in this organ. In addition, neontal thymectomy decreased the relative numbers of small lymphocytes in the lymph nodes. This was associated with drastic depletion of lymphocytes in the deep cortical regions of the nodes.     

Histogenesis of the immune system of the "nude" mouse. III. Postnatal development of lymph nodes and spleen: a light microscopical study (author's transl)]

In neonatally thymectomized mice, the intermediate cortical zones of the lymph nodes as well as the central portions of the spleen follicles (=thymus dependent areas, TDA) are depleted of lymphocytes within a few weeks (Parrott et al., 1966). The TDA's in the lymphatic tissues of mice homozygous for the gene "nude" (nu/nu) contain also a very small number of lymphocytes. Since only a few developmental stages of the lymphatic tissues in nude mice have been studied, an investigation of the postnatal differentiation of their lymph nodes and spleen seemed worthwhile. Materials and Methods: The mice (BALB/c) were maintained under specific pathogen-free (spf) conditions. Heterozygous (nu/ plus) females were mated with homozygous (nu/nu) males in order to obtain the homo- and heterozygous offsprings used in this study. Two nu/nu and two nu/ plus female mice were sacrificed at the following postnatal stages: day 1, 7, 14, 21, 35, 42, 49, 63 and 84, respectively. Newborn and 7 day old mice were fixed in toto in Bouin's solution and embedded in paraffin. From the other animals, spleen and mesenteric lymph nodes were removed and processed in an identical manner. Serial sections were cut at 5 mum and stained with hematoxylin and eosin. Results: The mesenteric lymph nodes of the nu/ plus mice studied were indistinguishable from those of other mouse strains. In the nu/nu mouse, outer cortex and medulla of the mesenteric lymph nodes were found to develop as in normal (nu/ plus) animals, while the intermediate and deep cortex underwent characteristic changes...     

The effect of long-term lymphatic drainage on the lympho-myeloid system in the guinea-pig

Methods are described for the long-term drainage (9 or 10 days) of the mesenteric lymph duct in the guinea-pig and for the analysis of bimodal nuclear distributions by electronic sizing of lymphocytes in lymph.

The present studies show that on the basis of nuclear volume, lymphocytes in mesenteric lymph consist of two populations (large and small) which are log-normally distributed. The relative proportions of the large and small lymphocytes and means and standard deviations of these component populations were estimated daily during the course of cannulation. The daily output of small lymphocytes was greatly reduced during the course of drainage to a base level of 110× 10⁶ on day 6 which was maintained thereafter. The daily output of large lymphocytes was relatively constant at about 50× 10⁶ throughout drainage. This finding is consistent with the view that many small lymphocytes are recirculating.

The output of cells in mesenteric duct lymph on the 1st day of cannulation (379× 10⁶) is similar to that recorded by other workers in thoracic duct lymph for the same period. Comparison of the present findings with the results of other studies shows that the level of `indirect entry' lymphocytes and lymphocyte blood concentration remain remarkably constant while blood volume may be doubled. It is suggested that `direct entry' of lymphocytes from lymphoid tissue to blood increases with age (body weight and, therefore, blood volume) of the animal.

During lymphocyte depletion the concentration of bone marrow lymphocytes was reduced from 466,500 to 173,500/mm³. There was no significant reduction in myeloid or erythroid cells. This finding suggests that either bone marrow lymphocytes are not involved in the production of erythroid and myeloid cells or that haemopoietic precursors are drawn from marrow lymphocytes produced in situ rather than from those which are haematogenous in origin.

Histological study of the lymphoid tissues of animals following long-term drainage showed a gross depletion of small lymphocytes in the cortex of nodes (particularly the mesenteric), white pulp of spleen and internodular areas of Peyers patches. The occurrence of large pyroninophilic cells with vesicular nuclei and prominent pyroninophilic nucleoli in depleted areas was noted. Mitotic figures were frequently observed in these cells. The thymus was not obviously affected by drainage.

     

An Anatomo-pathological Study of the Lymphoid System in Hamsters During the Growth of an SV40-induced Tumour

A systematic histological study of the lymphoid organs of the Syrian hamster was undertaken during development of a tumour induced with SV40 transformed cells. The essential features were: (1) A peritumoral plasma-cell reaction was present until the tumour reached a weight of 4-6 g; then it disappeared. (2) In the thymus an initial increase in mass with progressive infiltration of cortical cells into the medullary region was noted. Coincidentally with the disappearance of peritumoral plasma cells, thymus size descreased. As tumour growth proceeded, the medulla was progressively invaded by cortical and blood-borne cells which eventually made up more than 95% of the total population. (3) The spleen showed sequential changes. An early period of mild banal inflammation characterized by scattered immunoblasts in the reticulum was followed by a reduction in the number of lymphoid follicles, a loss of T lymphocytes in the periarteriolar areas and the appearance in the reticulum of young lymphocytic cells. This change coincided in time with disappearance of the peritumoral plasma cells. This change in turn gave way to a final stage characterized by progressive accumulation in the reticulum of young lymphoblastic cells which gave the spleen a pseudoleukaemic appearance and suggested a hyperimmune state. (4) The lymph nodes were the site of a mild plasmacytic stimulation in the region of the cortico-medullary junction; this persisted throughout tumour growth.     

Bovine lymphocytes: recognition of cells forming spontaneous (E) rosettes.

About 20% of thymus lymphocytes from neonatal calves formed spontaneous (E) rosettes with SRBCs in medium consisting of 50-100% foetal calf serum (FCS); other media were less satisfactory. FCS was necessary both to allow rosette formation to occur and to maintain stability of the rosettes once formed. Rosettes were stable at 0 degrees C but unstable at 18 degrees C and 37 degrees C. Dead thymus cell (sodium azide treated) did not form rosettes. Treatment of thymus cells with antiserum to bovine Ig-inhibited rosette formation, but this inhibition was considered non-specific since it also occurred with normal rabbit serum. Treatment of SRBCs with neuraminidase slightly enhanced rosette formation by thymus cells, but did not induce peripheral blood lymphocytes to form rosettes. Rosette formation did not occur under a variety of conditions with normal or neuraminidase-treated human, horse, pig, rabbit, guinea-pig, chicken or autologous RBCs. SRBC rosette forming cells were also found in lymph nodes (2-14%) and spleen (less than 5%), but rarely or never in peripheral blood and bone marrow of calves and adults. In foetuses at 80 days of gestation, 49% of thymus cells formed E rosettes. Foetal lymph node cells formed E rosettes at 160 days and spleen at 180 days. Cells with membrane-bound Ig were observed by IFT; their distribution did not coincide with the occurrence of E rosettes. E-rosette formation might be a marker for a subpopulation of bovine T cells.     

Lymphocyte Subpopulations in the Liver,Spleen, Intestines,and Mesenteric Nodes: An Immunohistochemical Study Using Human Fetuses at 15–16 Weeks

The roles of the liver and intestines in lymphocyte differentiation in human fetuses were assessed by immunohistochemical analysis of the thymus, bone marrow, liver, spleen, intestines, and lymph nodes of 15–16 week human fetuses using primary antibodies against IgM, CD3, CD7, CD8, CD10, CD20, CD45RO, HLA‐DR, and CD68. The density of immunoreactive lymphocytes was high in the thymus and lymph nodes, but much lower in the bones, liver, spleen, and intestines. The medulla of the thymus contained IgM‐positive mature B lymphocytes as well as CD20‐positve B lymphocytes. In contrast, CD10‐positive immature B lymphocytes were restricted in the cortex. There were no site‐dependent differences among axillary, mediastinal, mesenteric, and pelvic lymph nodes. CD68‐positive cells were observed at all sites examined. Many HLA‐DR‐positive round cells were present in the thymus, with fewer in the liver and spleen. The absolute number of lymphocytes was estimated to be ≥10‐fold higher in lymph nodes than in liver. Although limited by analysis of only one fetal stage, these findings suggest that mesenteric nodes are likely to be more important than the liver, spleen, and intestines for lymphocyte proliferation and differentiation in human mid‐term fetuses. Anat Rec, 297:1478–1489, 2014. © 2014 Wiley Periodicals, Inc.